1. Field of the Invention
This invention relates to a thermosetting polyurethane resin, an aqueous dispersed thermosetting polyurethane resin prepared by dispersing it in water and to coating agents comprising them. This invention particularly, relates to a one-pack type thermosetting polyurethane resin which possesses excellent mechanical strength, appearance, durability and water resistance as a surface-protective coating agent for beverage glass bottles which require sterilization with hot water.
2. Description of the Prior Art
With respect to a thermosetting polyurethane resin with high resistance to hot water, two-pack reactive type systems such as Desmodur/Desmophen by Bayer A.G. or a system by AULD Co. Ltd. are known. However, two-pack systems are difficult and costly to use.
One-pack type thermosetting polyurethane resin systems and one-pack systems which are water-dispersions thereof are known in the art. One-pack systems in which a blocked-isocyanate radical is employed are disclosed, for example, in Japanese Patent Publication examined No. 5989/1964, Japanese Patent Publication examined No. 11558/1978, Japanese Patent Publication examined No. 47519/1977, and Japanese Patent Publication not examined No. 105599/1978. However, when the prior art one-pack systems are used as a coating agent for glass bottles, the coatings do not have sufficient resistance to hot water so as to make sterilization of the coated bottles with hot water possible.
In U.S. Pat. No. 4,163,814 an aqueous dispersed polyurethane composition for coating glass bottles is disclosed. The film prepared from the composition, it is disclosed, has excellent mechanical strength, is waterproof, and does not lose its transparency when placed in hot water of 70.degree. C.
The present invention is the result of an extensive study to overcome the disadvantages associated with the two-pack systems and with the prior art one-pack systems.
The present invention involves a one-pack type thermosetting polyurethane resin and water dispersions thereof, with such a high resistance (i.e. high mechanical strength and maintenance of transparency) to hot water as to make possible the hot water sterilization of articles coated with the cured resin at temperatures of about 90.degree. C. The water-dispersions of the present invention can be substantially free of solvent.